Honestly GitHub - Redfire75369/spiderfire: JavaScript Runtime built with Mozilla's SpiderMonkey Engine may be a better starting place to experiment with a SpiderMonkey-based JS runtime. I don’t know much about it, other than that the docs look to be in an early state, and the author is knowledgeable, capable, and shows up frequently on the Matrix channel.
I asked Can js-compute-runtime be run completely locally? (SpiderMonkey JavaScript engine), though did not/do not see a clear instruction set for how to do that (without running some other software) in the linked documentation.